How to check if, lets say, list_1 in list_2 without iterating the list_1? Are there any specific 'pythonic' ways or it's better to stick to:
for i in list_1:
if i in list_2:
Thank you!
Asked
Active
Viewed 60 times
0
John Coleman
- 51,337
- 7
- 54
- 119
Valera Maniuk
- 1,627
- 3
- 11
- 14
-
I can think of at least 5 possible meanings of "list_1 in list_2". Would you care to share yours? – John Coleman Jul 27 '15 at 23:44
-
related: http://stackoverflow.com/questions/16579085/python-verifying-if-one-list-is-a-subset-of-the-other – NightShadeQueen Jul 28 '15 at 00:41
2 Answers
3
You seem to be interested in cases where every element in list_1 is also in list_2. In that case a simple
set(list_1) <= set(list_2)
works.
Of course -- if list_2 is a list of lists and you want to know if list_1 is one of those lists a simple
list_1 in list_2
works.
Alternatively, you could be asking if list_1 is a slice of list_2. Just check
list_1 in (list_2[i:j] for j in range(len(list_2)+1) for i in range(j))
You could also check is list_1 is a sub-multiset of list_2 or if list_1 is an increasing subsequence of list_2. I'll leave those as exercises.
John Coleman
- 51,337
- 7
- 54
- 119
-
Thanks! That is exatly what I tried to learn. I also tried a "subset" exercise, please let me know if there is a better way: subset_of = True while list_1: try: list_2.remove(list_1.pop()) except ValueError: subset_of = False break print(subset_of) – Valera Maniuk Jul 29 '15 at 04:52
-
1
Python has some functional support with map and filter which help in avoiding levels of iteration.
For iteration with inner conditions/early returns, the any and all functions are effective.
Also, set operations (intersect, union, difference) can be helpful too. It's probably what I would choose when just seeing if the elements in list 1 are in list 2.
list_1 = [ 2, 3 ]
list_2 = [ 1, 2, 3, 4 ]
print set(list_1) in set(list_2) # True
# a more functional approach, although really only makes sense for more specific complex examples!
import operator,itertools,functools
is_in_list_2 = functools.partial(operator.contains,list_2)
print all(itertools.imap(is_in_list_2,list_1)) # will hopefully not call contains more than it needs to!
parity3
- 643
- 9
- 18